Large-scale industrialization over the past century has led to a variety of expanding environmental impacts (e.g., air, soil and water pollution, deforestation, desertification), resulting in both urban and rural public health safety concerns and significant concern over human-driven planetary changes. Changes in climate include changes in precipitation distribution and an increase in average temperatures globally, significantly impacting plant and animal ecosystems in various ways such as e.g., plant growth rates, soil mineralization, metabolic rates, life-cycle changes and animal migration patterns. Due to their ubiquitous and resilient nature, plant health and growth rates have been used as indicators for various environmental factors. For agriculture applications, rapid plant monitoring has remained a subject of great importance for maintaining plant health and identifying potential emerging diseases, which can affect plant storage dynamics and crop production efficiency.
Mainstream efforts have focused on indirectly measuring plant chlorophyll concentration, which is considered an important metric for general plant health. Abnormal levels of chlorophyll may be indicators of important plant stress agents such as e.g., plant diseases, environmental stress, lack of or excess amounts of nutrients, light, or water, or the presence of toxic substances (e.g., cadmium). Currently, the “gold” standard for direct measurement of plant chlorophyll concentration is through chemical extraction of the chlorophyll from plant specimens, whereby plant leaves are mechanically dissociated and dissolved using chemicals (e.g., acetone). The chlorophyll is then filtered from the other plant compounds and pigments and chlorophyll levels are subsequently measured using a spectrophotometer. This process is inherently destructive, expensive, complex, and, due to its sample preparation steps requires trained personnel within a controlled lab environment (and proper equipment). To combat these disadvantages, over the past three decades several indirect methods for estimating chlorophyll levels of plants have been developed. Hyperspectral satellite imaging techniques using indirect chlorophyll estimation systems have been effectively used to control and monitor crop fields and forested regions, deforestation processes, and the spread of invasive species. While effective at the macro scale, these methods require complex and expensive hardware and time-consuming data acquisition and processing. In recent years, various hand-held optical systems (e.g. GreenSeeker™, atLeaf+, or SPAD 502Plus) have also been developed to form portable instruments for estimating the chlorophyll information of plants in a local region. One of the most reliable and most frequently used hand-held device is the commercially available SPAD-502Plus (Special Product Analysis Division) chlorophyll meter offered by Spectrum Technologies, Inc., which uses ratio-metric analysis of the light absorption of a leaf under a red LED (peak wavelength of 650 nm) and an infrared LED (peak wavelength of 940 nm) for estimation of site-specific chlorophyll concentration levels on the leaf. While widely used for agriculture and plant physiology studies, this device is relatively expensive and only provides an estimation of chlorophyll levels for a small (2 mm×3 mm) area on the leaf surface, necessitating multiple measurements across other areas of the leaf surface for estimation of the overall leaf chlorophyll concentration. There is a need for other hand-held devices and systems that can effectively measure chlorophyll levels in plant leaves.